Public address stabilizing system



May 9, 1939- Efw. KELLoGG 2,157,178

PUBLIC ADDRESS STABILIZING SYSTEM Filed Dec. 3l, 1936 2 Sheets-Sheet l'J4 5 BB y Cttorneg E. W. KELLOGG PUBLIC ADDRESS STABILIZING SYSTEMFiled Dec. 3l, 1936 Sheets-Sheet 2 loudspeaker.

Patented May 9, 1939 PUBLIC ADDRESS STABILIZIN G SYSTEM:A

Edward W. Kellogg, Moorestown,

N. J., assigner to Radio Corporationof America, a corporation y ofDelaware Application December 31, 1936, Serial No. 118,441

14 Claims.

l 'I'his invention relates to signal transmitting and receiving systems,such as are utilized for public address, sound reinforcement or likepurposes, and has for its principal object the provision of'an improvedapparatus and methodl of operation whereby undesired feed-back from theloudspeaker tothe microphone of such systems is avoided. l

Such feed-back between acoustically coupled sound reproducing andpick-upA devices, commonly results from sound which (1) is produced bythe loudspeaker, (2) is picked up by the microphone and (3) producestrical current which is amplified and fed to the If the microphone andloudspeaker are in the same room, the amount of sound amplificationywhich can be employed is practically always limited by the feed-backpoint which almost always occurs at the frequency at which the overallamplification is at a maximum. This involves the complete circuit,starting with the microphone and ending with the sound received bythemicrophone from the loud speaker, thus including suchsound reinforcementor sound losses as are imparted by the entire lacoustical environment.'Ihusl the microphone may be especially sensitive at one frequency orthe loud speaker may be especially efficient at a particular frequencyor the amplier may have a peak at a certain frequency. All these factorshave a part in determining the frequency at which feed-back becomesevident.

It has been lfound that the usefulness of a sound reinforcing system isgreatly enhanced by minimizing all of the peaks in thesystem so that theoverall characteristic is as nearly uniform With respect to frequency aspossible. Further advantage has been taken of the directive propertiesof both microphones and loudspeakers in order that the microphone mayreceive a minimum of sound from the loudspeaker as compared with thesound which the loudspeaker radiates toward the hearers, and the soundwhich the microphone receives from the original source. It has also beenproposed to provide the amplifier of a public address system with a slowacting automatic volume control of the compressor type for reducing theamplification when there is any prolonged sound. 'I'his would serve tostop feedback ifv the sustained sounds were due to feedback but nosatisfactoryarrangement has been proposed that will enable themicrophone to dis tinguish between feed-back sounds which it is desiredto amplify. In spite of what it has been possible to accomplish alongthese lines, the usea corresponding elecfulness of public addresssystems is still seriously limited by feed-back.

A possible solution of the problem is the provision of means for causingthe amplification to be momentarily increasd in response to speech.- For5 this purpose, a quick acting expander circuit having a limited rangeof volume control'may be employed. Such. a systemV is disclosed by aLeslie J. Anderson application, Serial No. 113,131, filed November 28,1936, and assignedl to the same assignee as the present application. Thetheory of this approach tothe problem is that the high amplificationcondition does not last long enough to permit the building up offeed-back oscillations.

A possible solution of the problem of enabling the control system todistinguish between currents resulting from feed-back and those due touseful sound is the provision of a system including 1) means forcausing. the overall amplication at a certain frequency to exceed thatat all other frequencies so that feed-back, if it occurs at all, will beat this frequency, and (2) means responsive to sustained currents ofthis frequency for reducing the gain at all frequencies to a level 26below that at which self-excited oscillations can be maintained. Such asystem is disclosed in my application, Serial No. 113,121, led November28, 1936 and assigned to the same assignee as the present application. v

The present invention Adiffers from those of the aforesaid applicationsin that it involves the provision of means for periodically shifting thephase of the loudspeaker output with respect to the microphone. As willhereinafter appear, this periodic' shift in the phase relation betweenthe loudspeaker output andthe microphone may be effected by (1)cyclically varyingthe length of the air path between the microphone andloudspeaker by moving the loudspeaker with respect l tothe microphone orvice versa, or (2) a cyclic or a continuous phase change introducedbetween the microphone and the loudspeaker. 'I'hese solutions of theproblem are based on thelfact that feed-back at any given frequency isdependent on a certainphase relation between the loud speaker output andmicrophone input, and any phase shift upsets the process of building uposcillations.

'I'he invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawings and its fscope is indicated by the appended claims.

Referring to the drawingscyclicaily varying the position of aloudspeaker, Figure 3 illustrates a mechanism for cyclically varyingphase by varying the length of an air column which is introduced in theamplifying system between the microphone and loudspeaker, Figure 4illustrates a system including an elec.- trical network for producing avariable-delay between the loudspeaker and microphone,

Figure 5 is'anblock diagram of a system in which the phase shift isproduced by causing a continuous drift of phase of all the frequenciesthroughout the audiolvrange, and

Figures 6 to 8 are wiring diagrams of different circuits which areuseful in producing the desired phase shift or frequency drift betweenthe loudspeaker output and microphone.

Figs.` 1' and 2 illustrate a possible mechanical arrangement for causinga loud speaker to move in a circular path. The loudspeaker is mounted ona triangular plate 'or frame 2 which is supported at the corners by arms3 which are plvoted at 4 and carry counter-weights 5. 'I'he pivots areof the ball and socket type, permitting the arms to move vertically orhorizontally. Ball and socket joints are also provided at 6. Theloudspeaker can therefore be readily moved either horizontally orvertically ,but is always pointed in the same direction. Motion may beimparted to the loudspeaker by means of a motor "I, which causesrotation of a crank pin 8. Two links 9 and I having ball and socket oruniversal joints at each end transmit rotational movements ofthe crankpin 8 to frame 2 and thereby to the loudspeaker.

Fig. 3 illustrates a method of introducing a. variable time lag betweenthe picking up of a sound by the microphone I2 and its reproduction bythe loudspeaker I. After amplification, the

output of the microphone I2 is applied to a re' I ceiver unit I4 whichproduces sound at one end of the air column in the tube l 5, I1. -Thesound produced by the receiver unit I4 travels the length of this tubeand is received by the transmitter I6 the output of which afteramplification is applied to loudspeaker I. The time lag between themicrophone |2 and the loudspeaker I is proportional tothe length oftheair column in the tube i5. This length may be varied by constructing thetube soqthat it may be telescoped as in the case of an ordinary slidetrombone. The U-shaped portion I'I of the tube I5 slides over the flxe'dstraight portions and may be caused toV move back and forth by means ofmotor driven crank I8 and connecting rod I9, a guide 20 being ,providedto take the transverse thrust of the connecting rod. The sliding andactuating arrangements must be so constructed as to introduce negligiblenoise in comparison with the useful sound infthe tube. Suitable dampingmust also be provided in order thatstanding waves may not be built upin'the tube. The transmitter unit I6, if of a high efilciencyelectromagnetic type, can itself afford the required damping. A suitablevent 2| must also be provided so that air may enter and leave the tube.The vent 2| should be aoustically damped.

Fig. 4 shows apparatus for producing variable delay in an electricalsystem. The output of the microphone 22 is amplified at 23 and suppliedto an artificial transmission line 24 constructed of suitably arrangedseries coils -and shuntv condensers. The waves lon this line areuni-directional reflections being prevented by'terminating the line witha resistance 25 of suitable value.

Figures i and 2 illustrate a mechanism for' throughout the audio range.

The voltage at various points along the line might be picked up byproviding a commutator device. or sliding contacts but I prefer topickup the magnetic field which surrounds the line by means of anexploring coil 26, which is mounted on a slide 21 and moved back andforth by the motor driven crank I9. The voltage generated in theexploring coil 26 is amplified by the amplifier 29 and applied to theloudspeaker I. The artificial line 24 is designed so that the .totaltime required by a wave to travel the length of the line is of the orderof one complete cycle of the lowest frequency for which there is anydanger of feed back. At the same time if line 24 is constructed oflumped capacities and inductances, there must be at least threecondensers per wave length of the highest frequency to be transmitted.`

Fig. shows a modification of the invention in which the change of phaseis produced by causing a continuous drift of phase of all frequenciesThis continuous drift, rather than a reversed shift, insures bettercancellation of the. standing wave patterns for all frequencies. 'I'hearrangement of Fig. 5 has the further advantage that no movingmechanical parts are required. .The output of microphone I2 is appliedto amplifier 32, the output of which is used in the balanced modulator33 to modulate a carrier frequency supplied by oscillator 34. 'Ihefrequency of the carrier should be well above the audio range, but thedesignsof the system is simplifled -if the frequency is not extremelyhigh.'

A frequency of the order of 20,000 cycles is suitable. The balancedmodulator is of a well-known type and results in the production of bothside bands, but with the carrier frequency absent.

g side band is transmitted consisting of frequencies equal to 20,000ycycles plus whatever frequency is present in the microphone output. Asecond oscillator 35is tuned to a frequency differing from that of 34 byonly a few cycles per second, for example, 20,002 cycles. 'Ihis secondcarrier current is mixed with the upper side band from the filter 3|.'I'he side band combined with the new carrier frequency is applied tothe detector 3l, amplifier 38 and loudspeaker I. 'Since we have assumedthe transmission of the upper side band anda new carrier higher infrequency by 2 cycles per second thanthe original, the audio currentssupplied to the loudspeaker will be lowerin frequency than the originalsby two cycles per sec'- ond. Therefore, the phase relations between thehelpful in a large auditorium than in a. small one.-

The reason for this is that the larger the auditorum or the greater thedistance, the longer it takes to establish the system of standing waveswhich is usually responsible for building up the pressures at themicrophone to the point at which vfeed back can occur.

other may be kept to a minimum. If, in addition to avoiding actualsynchronism, it is necessary to maintain the difference frequency withinnarrow limits, the problem is still more difficult. For example, if itis desiredvto operate two oscillators, one at 20,000 cycles and theother at 20,001 cycles, it is necessary that both `oscillators maintaintheir frequency within less than one part in l20,000.

If the difference in the two frequencies is large, a sum or differencefrequencycan readily be obtained by modulating an oscillator andeliminating the carrier and one side band by means of a filter. `Wherethe difference between the two desired frequencies is very small, nofilter can be designed which `will properly separate one side band fromthe carrier and other side band.

One of the possible methods `of obtaining an alternating current offrequency f2 from a frequency f1' is to supply a polyphase stator withfrequency f1 and rotate the secondary winding at such -a speed thatfz-fi pairs of poles are passed per second. In power work, transfonnersfor such service are sometimes caled Selsyn mo' tors or Selsyn'generators. In radio frequency work, an air core coil may be'employed,but the principle of operation is the same.

In Fig. 6, 4I -representsan oscillator supplying current of frequencyf1. 'Ihe converter 42 has two windings 43 and 44, so disposed as toproduce magnetic fields at right angles to`each other. 'I'he winding 43is supplied directly and the winding 44 through a delay circuit 45designed to cause the current inwinding 44 to lag 90 with respect tothat in Winding 43. Resistances 48 are included in the circuits toproperly load the lines so that the delay circuit will functionproperly. Coil 41 is mounted on a shaft, and can be rotated se that itwill alternately be in une with winding 4 3 and 44. The voltage inducedin coil 48 has a. frequency f3, equal to fri-fz, if coil 41 is rotatedf2 revolutions per second in one direction, or equal to fi-fz if coil 41is rotated in the opposite direction.

Fig. v'1 shows an all electrical system for accomplishing the samepurpose without the use of moving mechanical parts. In this arrangement48 is an oscillator producing alternating current of frequency f1,which, for purpose of illustration, we shall assume to be 20,000. Asecond oscillator l49 produces a low frequency by which the 'desiredsource is to differ from f1. This is taken for illustration as 5 cyclesper second, it being assumed that the desired frequency f3 is 20,005cycles. Two balanced modulators 50 and 5| are employed, by which the20,000 cycle output of oscillator 48 is modulated at the rate of 5cycles per second. The two modulators function in identical manner,except that both the high and the low frequency voltages applied tomodulator 50 are in quadrature with those applied to modulator 5l. Thequadrature relation of the 20,000 cycle voltages isi produced by meansof the delay circuit 52, and that of the low frequency or modulatingvoltage by means of the delay circuit 53. Resistances 54v and 55 areused to load the phase shift circuits. Each balanced modulator producesa current which has an average frequency equal to f1 but which rises tomaximum, falls to zero, rises to maximum in the opposite phase and fallsto zero again, with `anced modulators are added.

each cycle of the modulating frequency fz. If a voltage which ismodulated in this manner is added to another of the same frequency andamplitude, but in quadrature with the first, the rise and fall of thesecond voltage being also in quadrature with that of the first, theresultant voltage will have a frequency equal to that of the originalfrequency f1 plus or minus the modulating frequency, depending on howthe outputs of the b'al- The relationship as follows:

50 may be repremay be shown mathematically The output of modulatorsented as A .'(cos 21rf1t) (cos 21rf2t) and that of modulator 5I as A(sin Zrfrt) (sin 21u-fat) The sum of these ma. be shown to be y A cos2r(f1+fn) t and their difference to A cos 2r(f1-f2) t If the modulationrate is very low, as for example, 1 to 5 cycles per second, a delaycircuit to produce a quadrature phase shift is difficult to obtain.Under these circumstances, resort equal to may be had to the method ofproducing the low frequency voltage by means of beats between voltagesof higher frequency, and rectifying the sum of the higher frequencyvoltages.. Although some dimculty might be encountered in constructingtwo oscillators of. highfrequency, as for 4example 20,000 cycles orthereabouts which diifer in frequency by only a few cycles per second,it is easy to build two low frequency oscillators whose frequenciesdiffer by a very small amount. We may, for example, construct twooscillators for,20 and 21 cycles re pectively. and these could bewell'enough stabilized to maintain the one cycle beat-rate over a longperiod of time. If a phase shift is produced in either of the twovoltages which are beating together, a corresponding phase shift takesplace in the beat. A comparatively simple delay circuit will suillce toproduce a phase shift in a 20 cycle current.

. Fig. 8 shows a system including two oscillators 56 and 51 and fourdetectors 61, 68, 69, 10 in push-pull arrangement, which may be utilizedto obtain two sources of one cycle voltage which arrangements, and ifthe beats are in quadrature,

so likewise will be the one cycle currents which result fromrectification of the beating twenty and twenty-one cycle currents.

Since it would be dimcultlto construct a transformer to operate at 1cycle per second, the seesaw" or push-pull effect required for thebalanced modulators V65 and 66 of Fig. 8 is obtained by employing tworectiflers, one n each side, and the transformers 1|, 12, 13 and 14,through which the 20 and 21 cycle voltages are combined for applicationtothe rectiflers, are connected in opposite directions for the tworectifiers of a pair. so that when they are adding on one side they 81through transformer 1I willmbe subtractingv while those applied torectier through transormer12 wil1`be in fuli'addition." A half cyclelater the voltage applied to rectifier 81 will be the sum and thatappliedl to 88 the difference of the .10 20 and 21 cycle voltages. Thus.the rectiilers willvcause the grids of the balanced modulator tubes 15and 16 to alternately swing positive and negative at one cycle persecond. These tubes ha've non-linear characteristics, such that theiramplification of .the 20,000 cycle voltage supplied by oscillator 48varies with the slowly varying, mean grid potential. Thefr or 20,000cycle voltage is balanced out to produce zero output when the two tubes15 and 16 have the same grid bias, but as the balance is upset first inone direction, then in the other, at the rate of one cycle per second,there results a modulated high frequency output current as alreadydescribed in connection with Fig. 7, which is equivalent to the sum oftwo gg equal side bands but withoutthe carrier.

Circuits 19, llill, 8|V and 82 serve to filter out the 20 cycle voltageso that this will not be applied to the grids of the balancedmodulatortubes. Resistances are also provided in these four circuits to dischargethe voltages built up by the rectiflers when `the 20 cycle voltageapplied.

across the latter decreases. The filtered outputs of the rectiflers 81,88, 88, 10are one cycle currents, the 20 and 21 cycleA components havingbeen removed by the lters 19. 80, 8|, 82. Since, as already explained,the beats between the 20 and 21 cycle currents occurin quadrature in theupper and lower detector systems, the one cycle currents also are inquadrature. The circuits 1 8, Il, 8l and 82 also provide capacitorsthrough which the 20,000 cycle current may `ilow in order that thevoltages of these frequenciesmay be applied to the grids of themodulator tubes 15, 16, 11 and 18.-

As in the previous cases, it is necessary, in establishing exactquadrature relations, to load the delay circuits with suitableresistances. These resistances and the values of reactance in the delaycircuits should in each case be adjusted until the desired phase angleand equality of the' two components is obtained. Resistances 58,80,

8|, 82, 53 are for the purpose of providing theV necessary load.

The foregoing description of Fig. 8 shows how the one-cycle voltage canbe obtained and applied to the grids of the balanced modulator tubes,and how these function to provide two sources of high frequency current,the one in quadrature with the other, and likewise modulated inquadrature with the other. The outputs of the two balanced modulatorsare added in Fig.

i 8 in such a manner that either the 20,001 cycle or component it wouldordinarily be necessary to4 make minor balancing adjustments in theoutputs of the' two balanced modulators. This may be accomplished invarious ways, as for example, by adjusting the plate voltages. or elsegrid bias of one or the other modulator. f i

Havingfobtained from the circuit .shown in Fig. 6, Fig. 7 or Fig. .8. acurrent of l'frequency slightly greater or slightly less than. f1 thisnew source of high frequency is employed-in the circuit arrangement ofFig. 5, in place of the output of oscillatorl 35, while `the requiredcurrent of frequency f1, supplied in Fig. by oscillator 3l, would beytaken directly from .oscillatorA Il of Fig. 6, oscillator 4 8 of Fig.'1 or 48 of Fig. 8.

I claim as'xny invention:

1. The combination of acoustically coupled the sound waves at saiddevices.

3. 'I'he combination of acoustically coupled sound reproducing andpickup devices, an amplifier connected between said devices, and meansfor periodically varying the length of the sound wave transmitting pathbetween said devices.

4. 'Ihe combination of acoustically coupled sound reproducing and pickupdevices, an amplifier connected between said devices, and means forproducing a phase drift of the sound waves at one of said devices withrespectv to the sound 'waves at the other of saiddevices.

5. The combinationN of acoustically coupled sound reproducing and pickupdevices, an amplifier connected between said devices, and meansincluding a delay network and an exploring coil associated with saidnetwork for changing 'the phase relation between the sound waves at saidincluding a plurality of interconnected oscillation I generatorsoperated at different frequenciesl for varying the phase relationbetween the sound waves at said devices. a

'1. The combination.` of acoustically coupled sound pickup andreproducing devices, means yaffording a carrier wave for the audioimpulses delivered by said pickup device,l means for transmitting onlyone ofthe side bands as modulated by said impulses, means 'forsuperimposing 'on said modulated side band a second carrier wave of afrequency slightly different from that of said first carrier -wave,means for detecting said impulses, and means for supplying said detectedimpulses to said reproducing device.

8. The method of minimizing ,feed-back between acoustically coupledsound pickup and reproducing Vdevices which includes varying the phaserelation between the sound waves at said devices.

9. The method of minimizing feed-back be-f tween acousticaliy coupledsound pickup and reproducing devices -which includes cyclically varyingthe phase relation between the sound waves at said devices.

l0. The method. of minimizing feed-back between acoustically coupledsound pickup and reproducing devices whichdncludes periodically varyingthe distance between said devices. l

11. The nethod of minimizing `feedback between acoustically coupledsound pickup and reproducing devices which includes periodically varyingthe length of the acoustic air path of the waves between said devices.

12.,The method o1' minimizing tendency iso4 current, supplying a newcarrier current having diierent frequency from the original, anddetectingv or demodulating the resulting modulated high frequencycurrent.

14. 'Ihe combination of acoustically coupled sound reproducing andpickup devices, an amplier connected between said devices, and meansincluding 'a delay network and a reciprocating exploring coil-associatedwith said network for changing the phase relation between the sound l0waves at said devices.

EDWARD W. KELLOGG.

